Display technology is one of the key elements in the development of new portable devices, which today typically feature wireless connectivity for voice and data access—and that will include a display for viewing, for example, text, graphics and different types of multimedia. The displays of such portable devices need to be increasingly capable of reproducing high quality still images and also live video. Such devices include, for example, advanced mobile phones and portable Internet appliances.
Many portable products are dominated by the display—both physically and from the cost perspective. The fact is that almost all other electronic components in such devices are shrinking in size except the display. The use of a microdisplay-based system instead of a large size direct view display panel promises one possible way to get over these limitations. Microdisplay-based systems may be generally defined as systems where the image produced by an image source has to be magnified for viewing. Generally, such microdisplay-based systems are driven by small, high-resolution integrated circuit display chips, but other configurations are possible too.
Microdisplays offer designers a chance to increase the displayed image size and resolution, yet physically shrink the size of the image source itself. In many cases, the smaller the image source, the lower the cost. So, not only do microdisplays promise to lower system costs, but their physically smaller size will mean less bulky and heavy products and smaller power consumption, that is they will operate longer with the same battery source. With a microdisplay-based system high pixel densities may be achieved. Many direct view flat-panel displays for instance, produce full colour pixels at only 3-4 lines/mm. Many microdisplay-based systems can provide full colour pixels at 50-100 lines per mm.
Microdisplay-based systems can be generally divided into two classes: projection display systems and virtual display systems.
Projection display systems create a real image on a screen. Suitable imaging optics magnify and project an image that is created on a display chip embedded within a projector.
Virtual microdisplay-based systems also use imaging optics to magnify an image, but to create a virtual image instead of a projected real image. A virtual image is what one sees when looking in an electronic viewfinder of a digital video camera, for example. The virtual image appears to be larger and floating at some distance from the eye of the observer—even though it is created by a small sized integrated display chip acting as the image source. In other words, the viewer has the illusion of seeing the source image as if he/she stands a certain distance away in front of a larger display monitor.
Virtual displays, which are kept close to the eye, can be monocular or biocular. One type of virtual display is, for example, a Head Up Display (HUD), where the imaging optics are located somewhat further away from the eye.
An important and well-known aspect in virtual display devices, as also in many other optical systems, is the exit pupil diameter of the system. The diameter and also the location of the exit pupil are of considerable practical importance defining largely the overall usability of the virtual display device. In visual instruments, including the virtual displays, the observer's eye needs to be positioned at the center of the exit pupil located behind the optical system in order to see the image with full field-of-view. In other words, the exit pupil is like a window, through which the virtual image can be seen.
The distance between the location of the exit pupil and the last optical component, for example, the eye-piece lens of a visual instrument is called eye relief. The eye relief, together with the exit pupil, defines the freedom of observation, i.e. the volume where the observer's eye can be located with respect to the optical system.